As is known, most known five-axis numerical control milling machines are equipped with a rocking worktable, which substantially comprises a cradle-like supporting structure hinged at both ends to the machine frame to rotate, on command, about a first horizontal reference axis; and a circular faceplate fitted for rotation to the central body of the supporting structure to rotate, on command, about a second reference axis perpendicular to the first reference axis. The faceplate is obviously coaxial with the second reference axis, and is normally positioned on the supporting structure so that its top face is coplanar with the first reference axis.
Known worktables are also equipped with two drive devices: a first for rotating the worktable supporting structure, on command, about the first reference axis to vary the tilt of the supporting structure, and therefore of the faceplate, as required with respect to the floor; and a second for rotating the faceplate, on command, about the second reference axis to vary the orientation of the work on the supporting structure as required.
In the best worktables currently used, the faceplate has a central cylindrical guide pin extending inwards of the supporting structure, coaxially with the second reference axis; and the second drive device is defined by an electric so-called torque motor housed inside the central body of the supporting structure so as to be connected directly to the cylindrical pin of the faceplate, and by a hydraulic brake device for selectively preventing any rotation of the cylindrical pin about the second reference axis. More specifically, the rotor of the electric torque motor is fitted directly to the cylindrical pin of the faceplate, and the stator of the electric motor is housed inside the central body of the supporting structure so as to be fitted to the rotor.
Though unequalled in the performance of conventional machining operations requiring accurate position control of the work, worktables of the above type have serious limitations when the milling machine is called upon to perform machining operations typical of a lathe. In which case, contrary to standard practice, the milling machine may be called upon to rotate the work about the second reference axis at angular speeds well above 1000 rpm, while the tool removes material from the work while remaining stationary in space and resting on the surface of the work.
As such, the electric torque motor controlling the position of the faceplate is obviously called upon to operate well outside normal operating conditions, with all the drawbacks this entails.
In the case in question, being specifically designed for low-speed rotation and highly accurate positioning of the work, the electric torque motor controlling the position of the faceplate is unable to reach rotation speeds of over a few hundred rpm without generating severe mechanical vibration, which may even impair operation of the machine and cause irreparable damage to the motor.